The present invention relates to a bipolar plate for a fuel cell.
The invention also concerns a fuel cell device, in particular of the solid electrolyte type, comprising at least one of said bipolar plates.
The field of the invention may be defined as that of fuel cells, in particular fuel cells of the solid polymer electrolyte type.
Solid polymer electrolyte type fuel cells in particular, find their application in electrical vehicles which are presently the subject of many development programs in order to bring a solution to pollution caused by thermal engine vehicles.
With solid polymer electrolyte fuel cells playing the role of an electrochemical energy converter associated with an onboard energy tank, for example hydrogen or an alcohol, problems may be overcome, notably problems with motor vehicles, charging times and costs, related to the use of batteries in electrical vehicles.
The schematic assembly of a fuel cell for producing electrical energy is partly illustrated in enclosed FIG. 1.
A membrane of the ion exchanger type formed of a polymer solid electrolyte (1), is used for separating the anodic compartment (2) where oxidation of the fuel such as hydrogen H2 (4), occurs according to the equation:
xe2x80x832H2xe2x86x924H++4exe2x88x92,
from the cathodic compartment (3), where the oxidizer, such as atmospheric oxygen O2 (5), is reduced according to the equation:
O2+4H++4exe2x88x9243 2H2O,
with production of water (6), while the anode and cathode are connected through an external circuit (10). The thereby produced water flows between both compartments by electro-osmosis and by diffusion (arrows 11, 12).
The ionic conducting membrane is generally an organic membrane of the perfluorated ionomer type containing ionic groups which, in the presence of water, provide conduction of protons (9) produced at the anode by oxidation of hydrogen.
The thickness of this membrane is from a few tens to a few hundred of microns and it results from a compromise between mechanical resistance and ohmic drop. This membrane also enables separation of gases. The chemical and electrochemical resistance of the membranes generally provide battery operation over periods greater than 1,000 hours.
The bulk electrodes (13) placed on both sides of the membrane, generally comprise an active area (14) and a diffusion area (15). The active area comprises porous graphite covered with noble metal grains (16), such as platinum, and a thin coating of an ionic conducting polymer, with a similar structure to that of the membrane, provides ionic transport. The diffusion area (15) comprises a porous material hydrophobized by applying a hydrophobic polymer, such as graphite coated with PTFE. The hydrophobicity allows the liquid water to be discharged.
Protons produced at the anode, through oxidation e.g. of hydrogen at the surface of the platinum grains, are transported (9) through the membrane to the cathode where they recombine with ions produced by the reduction e.g. of atmospheric oxygen giving water (6).
The thereby generated electrons (17), may then power for example, an electric motor (18) placed in the external circuit (10), with water as sole byproduct of the reaction.
The set of membrane plus electrodes is a very thin assembly with a thickness of the order of a millimeter and each electrode is supplied with gases from behind, for example by means of a fluted plate.
The power densities obtained through this recombination and which are generally of the order of 0.5-2 W/cm2, if hydrogen and oxygen are used, require the combination of several of these bulk electrode/membrane/bulk electrode structures in order to obtain, for example the 50 kW necessary for a standard electrical vehicle.
In other words, a large number of these structures must be assembled, the elementary surfaces of which may be of the order of 20xc3x9720 cm2, in order to obtain the desired power, notably if the fuel cell is used in an electrical vehicle.
For this purpose, each set formed of two electrodes and a membrane, defining an elementary cell of the fuel cell, is thus positioned between two impermeable plates (7, 8), which, on the one hand, provide hydrogen distribution on the anode side and on the other hand oxygen distribution on the cathode side. These plates are called bipolar plates.
Bipolar plates used in fuel, cells must fulfil several functions; they should i.a. meet the following criteria or requirements:
provide mechanical resistance of the bulk electrodes/membranes sets in the assemblies of the filter/press type, while limiting the thickness to a few millimeters in order to obtain an overall volume of compatible cell, notably for application in an electrical vehicle;
provide electronic and thermal conduction between the successive bulk electrodes/membranes sets, by obtaining the largest possible electronic and thermal conductivities in order to limit ohmic drops detrimental to the cell""s operation (excessive heating) and efficiency;
provide gas tightness while withstanding thermal and electrochemical corrosion associated with the specific operating conditions for a cell;
integrate diffusion paths providing homogeneous distribution of supply gases on the electrodes;
integrate components for removing excess water;
integrate cooling components.
Gas distribution channels with a square or rectangular section (19), for example with a side of about 1 to 2 millimeters, are machined on the bipolar plates for gas distribution. These channels are for supplying electrodes with gas in the most uniform manner as possible as they are laid out so as to meander over the whole surface of the electrode. They also enable the produced water to be recovered and discharged outside. These channels usually consist of horizontal sections separated by 180xc2x0 downward bends.
In order to minimize pressure losses between the gas inlet and outlet and to avoid imposing a too strong pressure difference between both faces of the membrane, several channels may be positioned on a same bipolar plate or distributor plate, for example in parallel.
It has been noted that the performances of cells provided with such bipolar plates, including gas distribution channels with a square or rectangular section, were still unsatisfactory, in particular because the attained relatively low maximum voltage, which is about 0.5 V to 0.7 V cm2 while operating in H2/air, may be considered as insufficient.
Further, the delivered voltage exhibits strong instability over time, and it is impossible to maintain the highest voltage level over a long period without their occurring occasionally large and totally unexpected variations.
These problems apparently seem to be related to the flow of various gas and liquid fluids flowing in the fuel cell device, and, in particular, in the gas distribution channels of the bipolar plates.
On the other hand, the presently used bipolar plates are either in graphite impregnated with resins, or in stainless steel. In both cases, it is necessary to resort to lengthy, costly and complicated machining for forming gas supply grooves, channels or flutes for the bipolar plates.
It was thus demonstrated that the cost of these plates may account for about 60 to 70% of the total cost for existing prototypes, a large, if not essential portion of the cost of the plates being associated with their machining.
Thus, if during recent years consequent progress has been made and has provided a reduction in fuel cell costs, by reducing the amounts of platinum used on the one hand and on the other hand to a lesser extent, the manufacturing costs for the required membranes, substantial progress still remains to be made as regards the plates, providing simplified implementation, notably by suppressing machining operations, in particular for obtaining gas diffusion paths.
Such a simplification in their manufacturing leading to a reduction in the plates"" costs would have repercussions on the cost of fuel cells bringing them into a price range similar to that of a thermal engine.
Accordingly, there is a need for bipolar plates for fuel cells, in particular for solid electrolyte fuel cells with which, when they are used in such a fuel cell, enhanced performances may be achieved, notably as regards the attained maximum voltage, and which ensure that these high performances are maintained over time, without their occurring unexpected and random variations of the latter, for such bipolar plates which on the other hand have to meet i.a. all the already aforementioned requirements.
In addition, there is also a need for bipolar plates, which may be simply prepared, in a limited number of steps and at a lower cost and by means of a method which in particular limits the lengthy and costly machining operations which cancel out the cost of these parts and consequently of the fuel cells which comprise them.
Accordingly, the object of the invention is to provide bipolar plates for fuel cells which meet i.a. all the aforementioned needs, which do not have the drawbacks, disadvantages, defects and limitations of the bipolar plates of the prior art, and which solve the problems of the bipolar plates of the prior art.
The object of the invention is further to provide a method for manufacturing such bipolar plates for fuel cells which is i.a. simple and of a low cost and reduced duration.
Finally, the object of the invention is to provide a fuel cell device exhibiting enhanced performancesxe2x80x94notably in terms of attained maximum voltage and of stability of such performances over timexe2x80x94with notably lowered costs, whereby such a fuel cell device may, in particular, be used in an electrical vehicle.
This and still other objects are achieved according to the invention, by a bipolar plate for a fuel cell comprising on at least one of its faces, at least one flute able to form with the surface of an adjacent electrode, at least one gas distribution channel, wherein said distribution channel has a shape or geometry so that the liquid of the biphasic flow flowing in said channel may be moved away from said electrode surface.
Generally, said adjacent electrode surface is a substantially vertical surface.
Preferably, it is the transverse section of said channel which has a shape or geometry such that the liquid of the biphasic flow flowing in said channel may be moved away from the electrode surface.
The shape or specific geometry of the channels and, in particular, of the transverse section of the gas distribution channels according to the invention, moves the liquid away from the electrodes, or even preferentially enables the liquid to be moved towards areas away from the surface of the electrode, for example towards an area which may be defined as xe2x80x9cthe bottomxe2x80x9d of said flute or said channel and formation of a liquid film such as a water film against the surface of the electrode is thus prevented.
Thanks to the specific shape or geometry of the gas distribution channels according to the invention, a particular state of flow is obtained which is favorable to the cell""s performances. Among the many geometries possible for the flow channels, this specific geometry according to the invention is the only one which enables a particular type of flow to be obtained, in turn, among all the possible states of flow, wherein the flowing liquid is moved away from the electrode.
Surprisingly, this type of flow is the one with which, in particular, the best performances of the fuel cell may be achieved in a stable way, notably the highest voltage for the longest period, without any random and unexpected variations over time.
With the bipolar plate, according to the invention, all the aforementioned needs may be met and a remedy may be found for problems posed by the bipolar plates of the prior art, including gas distribution channels which have a square or rectangular section.
The bipolar plate, according to the invention, meets all the above mentioned requirements, which bipolar plates for fuel cells should meet.
With the bipolar plate, according to the invention, a higher maximum voltage may be obtained than in prior art fuel cells provided with square or rectangular section distribution channels.
This high voltage level is maintained for a long period without any notable variation in voltage over time, i.e. voltage delivered by the cell is permanently very high and has a large time stability without any random variation. The cell""s reliability is enhanced.
The bipolar plate according to the invention also provides the advantage of widening the range of operating parameters, such as the cell""s acceptable flow rates, as compared with plates provided with square or rectangular section channels, i.e. the setting of parameters for operating the cell is less delicate and provides a larger error or uncertainty margin.
According to the invention and preferably, both faces of said bipolar plate comprise at least one flute.
In a first embodiment of the bipolar plate, according to the invention, the angles of the transverse section of said channel are more open on the side of said channel formed by said electrode surface than on the side of said furthest channel away from said surface.
Preferably, in this first embodiment, the transverse section of said channel has substantially the shape of a convex quadrilateral comprising a first side formed by said electrode surface, a second side opposite to said first side, and two other sides, wherein the angles between said first side and each of said two other sides are more open than the angles between said opposite side and each of said two other sides.
In this geometry, the angles are more open at the level of the electrode than at the level of the areas where the liquid, such as water is to be lead to, as desired. The less open or acute angles which are therefore between the side opposite to the electrodexe2x80x94defining the xe2x80x9cbottomxe2x80x9d of the channel and of the flutexe2x80x94and each of both other sides, correspond to the most significant capillary effects and the liquid is preferably placed in these areas.
In this first embodiment, said convex quadrilateral is preferably a trapezium, the bases of which form said first sidexe2x80x94the electrode surfacexe2x80x94and said opposite sidexe2x80x94the bottom of the channel and of the flute.
In other words, said first side and opposite side are then parallel to each other.
Still preferably, said trapezium is an isosceles trapezium, i.e. the sides other than the bases are equal.
In a second embodiment of the bipolar plate according to the invention, the transverse section of said channel is square or rectangular and, at least one of the walls or faces of the channel, other than that formed by the electrode surface, includes at least one groove or notch. The three walls or faces, other than that formed by the electrode surface, may include at least one groove or a notch.
Or else, only the two xe2x80x9csidexe2x80x9d walls of the channel, i.e. the walls other than the wall formed by the electrode surface and the wall of the channel opposite to the latter, include at least one groove or notch.
Or else, further, only the wall opposite to the wall formed by the electrode surface (bottom of the channel) includes at least one groove.
The shape of the grooves or notches may be of any shape, but generally these are grooves or notches with a V section with an aperture angle preferably less to 90xc2x0.
The size of the grooves is generally small as compared with the size of the channel, i.e. the depth and/or the width or the grooves only represents e.g. xc2xc to {fraction (1/10)}, based on the size of the walls on which they are formed.
According to a particularly advantageous aspect of the present application, the specific geometry of the gas distribution channels of the bipolar plates, according to the invention, is able to facilitate integration of new functions into these plates.
These examples may, for example, comprise extraction and/or feeding means for redistributing the liquid, providing e.g. internal humidification of the gases. This function is facilitated by localizing the liquid, such as water, in areas, according to the invention, away from the surface of the electrode.
Said extraction and/or feeding means for redistributing the liquid comprise, for example, means for drainage and/or capillary upward flow comprising, for example, at least one porous component provided, for example, in or on the flute, used for distributing the gas in the area where the liquid is localized, i.e., an area away from the surface of the electrode, preferably, in or on the wall opposite to the wall formed by the electrode wall, and/or in or on at least one of the side walls of said flute.
The porous components may for example assume the shape of a plate inserted between the channels and used for redistributing water to the benefit of deficient areas, for example at the inlet of the cell, or for extracting the produced water from the cell.
The porous material may also assume the shape, for example of trapezium-shaped inserts.
The porous material may be a foam with open pores made of various materials, such as metal or even polymer materials.
The liquid extraction and/or feeding means may also comprise perforations or holes provided in at least one of the walls of the distribution flutes or channels, preferably in those portions of the walls of these flutes in contact with said porous component.
According to a particularly advantageous embodiment of the invention, the bipolar plate comprises a single stamped and bent plate comprising a succession of alternating flutes with the same depth on both sides of said unique plate.
Said flutes upon assembly of the stack of bipolar plates with electrodes (or rather EME assemblies: electrode/membrane/electrode assemblies) will thus define on both sides of said unique plate, two respectively anodic and cathodic gas distribution channels or areas.
The invention also relates to a method for manufacturing the bipolar plates described above. The method according to the invention which is simple, quick and of high accuracy, consists of the succession of the following steps:
providing one or more wires of an adequate shape in order to define the flutes of the bipolar plate;
fixing said wire(s) on at least one of the faces of a planar plate in order to define one or more flutes with the desired geometry.
Preferably, said wire is obtained by extrusion through a die of an adequate shape and the wire(s) are fixed to the plate by spot welding.
The invention finally relates to the fuel cell comprising at least one bipolar plate according to the invention.